Spectrometer

ABSTRACT

The present invention relates to a spectrometer comprising a plurality of arrays of quantum dots, the arrays being arranged such that primary light emitted by a light source is incident thereon and excites the quantum dots such that the quantum dots emit secondary light such that the secondary light can be incident on a sample, the secondary light having a different spectral distribution to the primary light, at least one detector configured to receive the secondary light reflected or transmitted by the sample, and an evaluation device configured to determine spectral information of the secondary light received by the at least one detector. The invention also relates to a method for producing such a spectrometer.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a U.S. National Phase application under 35U.S.C. § 371 of International Patent Application No. PCT/EP2021/081056filed Nov. 9, 2021, which claims priority of German Patent ApplicationNo. 10 2020 216 283.2 filed Dec. 18, 2020. The entire contents of whichare hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to a spectrometer.

BACKGROUND

Spectrometers are used to determine the properties of various differentobjects. For instance, when analysing the chemical composition of e.g.everyday objects, food, waste, materials, raw materials, recyclablematerials, arable soils and pharmaceuticals, an absorption or reflectionspectrum needs to be determined. Spectroscopy furthermore offersnumerous possibilities in medical diagnostics. In particular thenear-infrared (NIR) wavelength range is highly relevant since inter aliacharacteristic and material-specific overtone and combination vibrationsof molecular components occur in this range. The analysis of theabsorption and reflection spectra defined by these vibrations inter aliaallows conclusions to be drawn regarding the origin, degree of ripenessand quality of food or regarding the composition of the samples to beanalysed, such as raw materials. Here and in the following, the term“absorption spectrum” is understood to mean a transmission spectrum,i.e. a spectrum of the light transmitted through the sample.

In the present application, the term “spectrum” refers to a distributionof the amplitude of the reflected or transmitted light as a function ofthe wavelength or frequency of this light. A wavelength-dependent orfrequency-dependent resolution of the amplitude is hereby possible. Inthe context of a spectrometer, the wavelengths emitted by the lightsource used are thus resolved in the wavelength or frequency range. Thereflection or transmission intensity to which the incident wavelengthscorrespond can accordingly be determined, although possibly notcontinuously but only for individual points.

Such absorption or reflection spectra are obtained by measuring theattenuation of the transmitted or reflected electromagnetic radiation ofone or more specific wavelengths by the sample material. Conventionalspectrometers are based on an adjustable monochromatic light source anda detector. The adjustable monochromatic light source usually consistsof a broadband light source, such as a halogen or deuterium lamp, and amonochromator. The latter usually consists of one or more diffractiongratings and slits, whereby selection of the wavelength can be achievedby rotating the grating(s). However, the use of mechanical componentsmeans that it is difficult to miniaturise such a spectrometer.

In order to achieve miniaturisation, the use of a wavelength-selectivedetector array is, for instance, proposed. The pixels of the detectorarray are hereby provided, for example, with different filters that aretransparent to different wavelengths. The transmitted or reflected lightintensity at different wavelengths can thus be determined. Suchspectrometers are described, for example, in US 2014/0061486 A1 or inU.S. Pat. No. 10,066,990 B2.

A further approach is to use a matrix of switchable light sources ofdifferent emission wavelengths. These may be, for example, differentLEDs, the emission wavelengths of which result from the use of differentsemiconductor materials. The use of colour converters/phosphors toadjust the LED wavelength is also possible. Such methods are described,for example, in U.S. Pat. No. 10,458,845 B2, JP 2008/020380 A, U.S. Pat.No. 8,279,441 B2, US 2012/0327410 A1, and U.S. Pat. No. 7,839,301 B2. Asimilar method is also described in U.S. Pat. No. 10,041,833 B1.

Although the determination of spectra was mentioned above, a furtherfield of use of spectrometers is the determination of spectralinformation. In the present application, “spectral information” isunderstood to mean that the wavelengths or frequencies incident on thesample may not necessarily be resolved in the output. For example, whendetermining the water content of a sample, it may be sufficient tomeasure the cumulative light absorption at a wavelength of 1950 nm and1450 nm in order to draw conclusions about the water content of thesample. The term “spectral information” is inasmuch to be understood tomean properties that are based on measurements of the transmission orreflection properties of the sample, but do not necessarily enablewavelength or frequency resolution. The determination of a spectrum isthus a specific case of determining spectral information.

SUMMARY

The present invention relates to a spectrometer that can be used toobtain absorption and reflection spectra or such spectral information bymeasuring light reflected or transmitted by a sample material. Thespectrometer is configurable and easy to manufacture.

The invention is defined by claim 1. Preferred embodiments are definedin the dependent claims.

According to claim 1, a spectrometer comprises a plurality of arrays ofquantum dots. Quantum dots (QDs) are nanoscopic material structurestypically consisting of one or more semiconductor materials. Chargecarriers (i.e. electrons and/or holes) have limited mobility in at leastone spatial direction in QDs due to the small particle size. Thislimitation leads to a change in the optoelectronic properties, inparticular the absorption and emission behaviour (“quantumconfinement”). Quantum dots typically contain in the order of 103 to 105atoms. Quantum dots may have various shapes, such as spheres with adiameter of 1 to 200 nm, preferably 2 to 100 nm, more preferred 2 to 50nm. Shapes such as rods, tetrapods, nanowires or platelets are alsopossible. In these cases, at least one dimension must be subject toquantum confinement. In the case of core/shell structures, at least onedimension of the core must be subject to quantum confinement.

These multiple arrays of quantum dots function as colour converters andare arranged in the beam path of primary light such that primary lightemitted by a light source can be incident thereon and excites thesequantum dots with the result that the quantum dots emit secondary light.In other words, the quantum dots convert light emitted by a lightsource, whereby the spectral distribution of the light changes in thatexcitonic states in the quantum dots are excited and subsequently relaxto emit secondary light. The secondary light has a different spectraldistribution to the primary light. This secondary light can be incidenton a sample and can then be used to determine the transmission orreflection behaviour of said sample such that the absorption orreflection spectrum thereof or, more generally, transmission orreflection spectral information can be obtained.

The spectrometer furthermore comprises at least one detector. Such adetector may, for example, be a photodiode or a photoconductive sensor(e.g. InGaAs, InAs, InSb, PbSe, PbS) or, for example, a CCD arraycapable of receiving secondary light reflected or transmitted by thesample. However, any other device that can receive the emitted light andthat can be used to detect the intensity of the reflected or transmittedsecondary light in a specific wavelength range is possible.

An evaluation device is furthermore provided, which is configured todetermine spectral information of the secondary light received by the atleast one detector. When recording a spectrum, this does not have to becontinuous—it is sufficient if intensity values are determined for oneor more discrete wavelengths.

The use of quantum dots to convert the light of a primary light sourceis advantageous in that it has been found that the emission wavelengthof the quantum dots, and thus the conversion behaviour thereof fromprimary light to secondary light, can be almost infinitely adjusted bythe choice of synthesis parameters, the composition thereof and inparticular the particle size thereof. Such quantum dots that differ fromone another in terms of spectra mean that it is possible to excitedifferent wavelength ranges of the sample and thus to determine spectralinformation/a spectrum over a large range. As a rule, quantum dots canalso be excited by photons of a broad wavelength range above theirbandgap energy, but exhibit narrow-band emission, the wavelength ofwhich corresponds approximately to the bandgap energy. This leads to ahigh degree of flexibility in the selection of suitable primary lightsources for exciting the quantum dots.

Formulations of different quantum dots, optionally with suitable matrixmaterials, furthermore behave very similarly with respect to theirdeposition properties, as a result of which a high degree ofconfigurability and flexibility is achieved when manufacturing thespectrometer. Quantum dot colour converter arrays with pixels ofdifferent emission wavelengths can thus be obtained in a cost-effectivemanner by applying different quantum dot segments. QDs of differentsizes can furthermore be mixed so that individual pixels can also beobtained, the emission spectra of which contain, for example, multiplebands and are matched to the absorption spectra of any targetanalytes/mixtures.

Such a spectrometer can therefore be easily and flexibly configuredduring manufacture, thus allowing for greater flexibility. QDsfurthermore usually have a high photoluminescence quantum yield and aresuperior to organic fluorophores, in particular in the NIR range. Highconversion efficiencies and light intensities can thus be obtained byusing QDs.

The spectrometer advantageously also comprises a light source arrangedsuch that the primary light emitted thereby is incident on the arrays ofquantum dots. While it is, in principle, possible for the spectrometerto use ambient light such as sunlight as its primary light, it isadvantageous for the spectrometer to comprise a light source. Thisallows the spectrometer to be used regardless of whether ambient lightis present and, if so, what type of ambient light is present. Such alight source also has a defined emission intensity and can be configuredsuch that it is particularly suitable for exciting the quantum dots. Ifambient light is used, a device is required to selectively admit ambientlight to the arrays of QDs or, if all or a plurality of arrays of QDsare illuminated simultaneously, to selectively admit the emissionthereof to the sample. Examples of such devices are LCDs, shutters ormicro-opto-electro-mechanical components. These can be disposed betweenthe array of QDs and the sample, or between the light source and thearrays of QDs, and thus selectively prevent light from the respectivearray from being incident on the sample.

It is furthermore preferred that at least two of the plurality of arraysof quantum dots have different emission spectra. This has the advantagethat not only the transmission or reflection spectrum at a singlewavelength is determined, but that this spectrum is also determined fora plurality of wavelengths (spectra recording). Such a transmission orreflection spectrum is significantly more informative. Differentemission spectra is understood to mean that the peaks of the maximumintensity of the spectra are shifted relative to one another by at least1 nm, preferably at least 10 nm, more preferred 50 nm, even morepreferred 100 nm and most preferred 200 nm. Alternatively oradditionally, the standard deviation of the peaks of the maximumintensity differs by at least 50 nm, preferably 100 nm.

It is furthermore preferred that the spectrometer is configured suchthat only one of the arrays of quantum dots with different emissionspectra at a time emits secondary light such that it can be incident onthe sample. In other words, this means that only one of the arrays leadsto secondary light being incident on the sample. This allows the sampleto be selectively illuminated with secondary light from one of thearrays, resulting in the ability to study the spectral behaviour of thesample relative to different excitation wavelengths/ranges. It is alsopossible for the light emitted by the individual arrays to be modulatedsuch that they are no longer controlled in a “binary”, i.e. on-off,manner, but in an analogue manner. It can in particular be ensured thatthe light is modulated in a wave-like (e.g. sinusoidal) manner.

It is furthermore preferred that the light source comprises a pluralityof sub-light sources, each of which is respectively coupled to one ofthe arrays of quantum dots such that light emitted by the respectivesub-light source is incident (only) on the respective array. Thus, byswitching the sub-light sources on or off or by modulating the sub-lightsources, one of the arrays of quantum dots can be excited in a targetedmanner. For this purpose, the control device is configured such that itcan control from which of the sub-light sources primary light isemitted, as a result of which the described controllability can beachieved. Power consumption can thus also be reduced since light sourcesdo not have to be operated unnecessarily. If, for example, LEDs or laserdiodes, which can be easily operated in a pulsed mode or in an otherwisemodulated mode, are used as primary light sources, a pulsed or modulatedemission of the QD arrays can thus be realised very easily.

Alternatively or additionally, the light source (11, 111) may comprise aplurality of selectively light-transmissive windows. These arecontrolled by the control device such that they selectively allow lightfrom a (single) primary light source to pass through. In this case, theprimary light source is a light source that provides light to some(preferably: all) of the windows, which is then incident on the arrays.The light is allowed to pass through the windows and then impinges onlyon the corresponding array, exciting the QDs thereof. It is alsopossible to provide a plurality of selectively light-transmissivewindows that are controlled by the control device such that theyselectively allow secondary light from the arrays to pass through, thesecondary light being generated by primary light from a single primarylight source. The windows can be implemented by means of LCDs orshutters or micro-opto-electro-mechanical members. Only a single lightsource is thus required, as a result of which the complexity of thearrangement can be simplified.

As an alternative embodiment, it is preferred that the light source becompletely external and independent of the spectrometer. For example,the light source may be ambient light or sunlight. It is, of course,also possible to use a specialised lamp as the light source. The primarylight emitted by the external light source can then be incident on thearrays.

The spectrometer then furthermore comprises a device that controls fromwhich of the arrays secondary light is incident on the sample. Such adevice may, for example, comprise components configured to selectivelyprevent primary light from the light source from being incident on oneor more of the arrays. As a result, it is possible to control on whichof the arrays primary light is incident and which of the arrays is thusexcited. Spectral information can thus be obtained by this selectivecontrol of the arrays. Alternatively or additionally, the device maycomprise components configured to selectively prevent secondary lightemitted by the arrays from being incident on the sample. Thus, incontrast to the embodiment described above, it is not the primary lightbut rather the secondary light that is blocked. This also allowsspectral information to be obtained. The components for blocking theprimary light and/or the secondary light preferably comprise shuttersand/or LCD members and/or micro-opto-electro-mechanical members. Suchcomponents are easy to implement.

With a spectrometer that uses an external light source and thus does notrequire a built-in light source, the cost and energy consumption of thespectrometer can be reduced.

It is preferred that the particle sizes and/or particle compositions ofthe quantum dots in at least two of the plurality of arrays of quantumdots differ from one another. By using different particle sizes and/orparticle compositions, the emission behaviour can be well controlled.

It is furthermore preferred that quantum dots having differing emissionspectra are present in at least one of the plurality of arrays. Thisallows the emission of these arrays of quantum dots to be adapted, forexample, to the absorption behaviour of analytes.

It is preferred that at least some of the quantum dots are configuredsuch that they emit secondary light in the near-infrared range. Theemission of such secondary light in the near-infrared range is of highpractical relevance, as explained above.

The wavelength of the primary light is preferably shorter than that ofthe secondary light. Such a down-conversion is of particular practicalrelevance since in particular secondary light in the near-infrared rangecan be effectively produced as a result thereof.

The quantum dots are preferably embedded in a matrix. Such a matrix mayconsist, for example, of the ligands of the nanoparticles and/or ofcompounds suitable for cross-linking quantum dots. The quantum dots canalternatively/additionally preferably be incorporated in an organicmatrix (e.g. a polymer matrix) or a matrix made of an inorganicmaterial. This is particularly advantageous for the application ofquantum dots.

Ligands of the quantum dots may, for example, be molecules carrying oneor more functional groups with which they can bind to the surface of thequantum dots. Examples of such functional groups are thiols,disulphides, amimes, phosphines, phosphonic acids, carbamates,thiocarbamates, dithiocarbamates, carboxylic acids, polyethers,phosphine oxides, dihydroxyphenyl groups and nitrile groups.

Suitable compounds for cross-linking quantum dots include molecules thathave a plurality of these functionalities and are sterically suitablefor binding to more than one quantum dot. By selecting a suitablecross-linking compound, composites of quantum dots can be produced, theproperties of which, such as the distance between the quantum dots, canbe adjusted. Inorganic compounds such as metal chalcogenides can also beused as ligands/crosslinkers [Kovalenko et al. Science 2009, 324, 1417].

A wide variety of organic substances, such as polymers, which have hightransparency to the primary light and the secondary light, can be usedas the organic matrix material for incorporating the quantum dots.Examples include photoresists (e.g. SU-8), (UV-curable) adhesives suchas Norland Optical Adhesive (NOA60, NOA61, NOA63, NOA65, NOA68, etc.),dendrimers, mercapto esters, thioesters, dithioesters, polythioesters,polydithioesters, polythiols, polythioethers, polymethyl methacrylate(PMMA), polystyrene (PS), polycarbonates, polyethylene terephthalate,polyurethanes, polypropylene, polyethylene, polyamides, polyethyleneglycol, polylactides, polyimides, polyisoprene, polyethers, polyesters,as well as copolymers or mixtures thereof.

Inorganic compounds that are transparent to the primary and secondarylight can be used as inorganic matrix materials. Examples includesilicones, aluminium oxide, titanium oxides, silicates, indium tinoxide, silicon dioxide, so-called spin-on glass, zirconium dioxide,sodium fluoride, sodium yttrium fluoride, lanthanum phosphates,lanthanum phosphorus vanadate, lanthanum vanadate, yttrium vanadiumphosphate, hafnium oxide, HSQ coatings, polydimethylsiloxane (PDMS),yttrium oxide, zinc oxide, silicon nitride as well as mixed phases.

Photostructurable (light-curable or light-destructible) matrix materialssuch as photocrosslinkable ligands or photoactive crosslinker molecules,suitable photoresists, or UV-curable adhesives have proven to beadvantageous. These materials allow an easy (direct) photolithographicstructuring of the arrays of quantum dots. Methods such as masking bymeans of photoresists followed by lift-off processes or etchingprocesses can be used for structuring other materials.

To apply the arrays of quantum dots, formulations of quantum dots andmatrix materials can be prepared, for example by introducing solvents.After deposition of the formulation, the solvent can then be removed, ifnecessary, to obtain the arrays of quantum dots.

The quantum dots can furthermore be introduced into formulations withprecursor compounds of the matrix materials. After deposition, theprecursor compounds of the matrix materials can produce the matrixmaterial and thus the arrays of quantum dots by means, for example, ofphoto-induced or heat-induced reactions.

By selecting a suitable matrix material, optimal deposition properties(e.g. viscosities and processing temperatures of formulations/inks,compatibilities with surfaces, adhesion to surfaces) can be set for theuse. In addition, the mechanical characteristics of thecoating/composite can be adjusted by the selection of the matrixmaterial. The matrix can in particular also protect (e.g. against(photo-)oxidation) the quantum dots against external influences (e.g.air, water).

The arrays of quantum dots are preferably arranged on a surface of thelight source. Such a configuration is particularly advantageous sincethe positioning of the quantum dots relative to the light source is nowfixed and can no longer change. The quantum dots, e.g. embedded in amatrix material, can thus be applied, for instance, directly to the LEDchip or the LED housing.

It is furthermore possible to arrange the arrays of quantum dots on atransparent or non-transparent substrate, e.g. a glass substrate or awafer, by means of lithography processes. This substrate can then, forexample, be arranged as a multispectral colour conversion filter on anarray of primary light sources or in the beam path of an array ofprimary light sources.

In addition, a suitable matrix material (e.g. a polymer) can itself beused as a support for the arrays of quantum dots. Quantum dot-containingfilms can, for example, be manufactured without a further substrate,which are placed as quantum dot-based colour converters on an array ofprimary light sources or in the beam path of an array of primary lightsources.

It is furthermore preferred that the quantum dots in one or more of thearrays consist of a plurality of material domains, the material domainspreferably having a core-shell configuration or a Janus configuration.Such quantum dots have proven to be particularly advantageous for use inspectrometers as emitter materials. Examples of core-shellconfigurations in quantum dots are described, for example, in [Jang etal., Chem. Commun. 2017, 53, 1002 to 1024] or WO 2014/033213 A2, thelatter in particular disclosing manufacturing methods for suchparticles. Such core/shell quantum dots can consist, for example, ofCdSe/CdS, ZnSe/ZnS, ZnTe/ZnSe/ZnS, CdSe/CdS/ZnS or InP/ZnS, InP/ZnSe,InP/ZnSe/ZnS and other combinations for the visible wavelength range orof PbSe/CdSe, PbSe/CdSe/CdSe [J. Am. Chem. Soc. 2017, 139, 32, 11081 to11088], PbSe/CdSe/CdS, InAs/ZnS, InAs/ZnSe, InAs/ZnSe/ZnS, InSb/ZnS,InSb/ZnSe, InSb/ZnSe/ZnS, PbSe/PbS, PbTe/PbSe, PbTe/PbSe/PbS, PbS/PbO,PbSe/PbS/PbO, PbTe/PbSe/PbS/PbO and other combinations for the NIRwavelength range. It may additionally be advantageous if the core/shellmaterials do not have discrete transitions, but rather mixed phasesbetween material domains. The particles may furthermore carrypassivating shells made, for instance, of insulating materials such asoxides, e.g. silicon dioxide.

The primary light is incident on the plurality of arrays of quantum dotspreferably in a modulated mode, even more preferred in a pulsed mode.Such modulated or pulsed incidence of the primary light can increase thesensitivity of the spectrometer since the use of a lock-in amplifier ispossible. The primary light that is incident on different arrays ofquantum dots may be modulated with different frequencies so that thesecondary light emitted by these arrays of quantum dots is alsomodulated with these different frequencies. This has the advantage thatlight from different secondary light sources that is reflected ortransmitted by the sample has modulations with different frequencies,and that the analysis of the spectral behaviour of the sample can thustake place by differentiating between the intensity signals measured inthe frequency domain at the detector. A sinusoidal modulation has provento be particularly easy to implement and analyse.

The quantum dots provided in at least one (preferably: all) of theplurality of arrays of quantum dots are preferably identical to oneanother. This leads to a great homogeneity of the secondary light whichthey emit. The term “identical” is understood here to mean that they areidentical in terms of size and composition to the extent allowed byquantum dot manufacturing processes within manufacturing tolerances.

Individual arrays of quantum dots may furthermore also include mixturesof quantum dots of different sizes and/or compositions. This has theadvantage that the spectral distributions of the light emitted by thesearrays of quantum dots may contain a plurality of emission maxima thatcan be adapted to the absorption/reflection spectrum of one or moretarget analytes. As an extreme case, arrays of quantum dots emitting abroadband NIR spectrum can thus also be produced. Such light sources aredifficult to realise in a conventional manner, e.g. with direct-emittingsemiconductors.

The quantum dots in at least one of the arrays are preferably made froma semiconductor. This is preferably a IV-VI, II-VI, III-V or I-III-VIsemiconductor and even more preferred a IV-VI semiconductor. Suchsemiconductors have led to particularly good conversion behaviour. Evenmore preferred, it is a semiconductor made of lead chalcogenides. Othermaterials that may be used for secondary light in the near-infraredrange are PbS, PbSe, PbTe, InAs, InSb, InAsSb, InAsP, CuSe₂, CdTe, HgTe,SnTe, SnSe, InP, Cu_(x)In_(y)Te_(z), Cu_(x)In_(y)Se_(z),Cu_(x)In_(y)S_(z), CuInSe₂, CuInS₂, Ag_(x)In_(y)Te_(z),Ag_(x)In_(y)Se_(z), Ag_(x)In_(y)S_(z), AgInSe₂, AgInS₂, and forsecondary light in the visible range are SnSe, SnS, CdSe, CdS, ZnS, InP,Cu_(x)In_(y)Se_(z), Cu_(x)In_(y)S_(z), CuInSe₂, CuInS₂,Ag_(x)In_(y)Se_(z), Ag_(x)In_(y)S_(z).

It is furthermore preferred that a method for producing a spectrometeraccording to any one of the preceding claims produces the arrays ofquantum dots by way of spin coating, dip coating, drop coating or aprinting process. It is particularly preferred, in the case of anarrangement of arrays of quantum dots to be produced, that spin coatingand dip coating be performed in conjunction with lithographicstructuring. Such methods are well characterised and established, andthus lead to a homogeneous, controlled deposition and to a spectrometerwith good properties as regards spectral analysis.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows a spectrometer according to a firstembodiment of the invention.

FIG. 2 shows the emission behaviour of quantum dots.

FIG. 3 shows a more detailed configuration of the spectrometer of FIG. 1.

FIG. 4 schematically shows a spectrometer according to a secondembodiment of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 and FIG. 3 schematically show an absorption spectrometer 10according to a first embodiment of the present invention. A light source11, which in the present embodiment takes the form of four sub-lightsources 11′ in the form of LEDs, is provided in this absorptionspectrometer. It should be noted that the number of sub-light sources isnot limited. Deflectable lasers or VCSELs could, for example, also beused instead of LEDs. This light source 11 emits primary light in awavelength range from UV to near-infrared light, i.e. a wavelength inthe range from 200 to 3500 nm, preferably 300 to 2000 nm, more preferred350 to 1100 nm.

The primary light emitted by these sub-light sources 11′ is incident onarrays 12 of quantum dots arranged at the sub-light sources 11′. As isschematically shown in FIG. 1 , the quantum dots in the arrays aresubstantially identical, but differ in diameter and thus have differentcolour conversion behaviour. The quantum dots in the arrays 12 are thusexcited by the primary light from the light source 11 and then relax toemit secondary light 13. The secondary light 13 typically has a longerwavelength than the primary light, which is referred to asdown-conversion. However, it is, in principle, also possible for anup-conversion to take place and for the secondary light 13 to thus havea shorter wavelength than the primary light. The wavelength of thesecondary light 13 is typically in the range of 300 to 5000 nm,preferably 390 to 3500 nm, more preferred 450 to 2500 nm. Owing to thedifferent particle sizes of the quantum dots in the arrays 12, thewavelengths λ₁ to λ₄ of the secondary light 13 emitted by the arrays 12differ.

The dependence of the wavelength of the emitted secondary light 13 onthe size of the quantum dots is also shown in FIG. 2 . This figure showsemission spectra of CdSe-based quantum dots (dashed lines) and ofPbS-based quantum dots (solid lines), each for different particle sizes.The Y-axis is normalised so that the maximum value of the emission is 1in each case. As is apparent from FIG. 2 , the emission behaviour of thequantum dots is strongly influenced by changing the particle size.

The secondary light 13 is then incident on the sample 21 to be analysed.The light 31 transmitted by the sample is then detected by the detector32. By switching the sub-light sources 11′ on or off in a targetedmanner, it is thus possible to determine the absorption behaviour of thesample 21 with respect to secondary light 13 having differentwavelengths λ₁ to λ₄, as a result of which spectral information or, inparticular, the transmission spectrum of the sample 21 can bedetermined. This makes it possible to determine the properties of sample21. The detector 32 may be a broadband detector using a semiconductorsensor, whereby such a semiconductor sensor may be based, for example,on CdS, CdSe, PbS, PbSe, InAs, InGaAs, InSb, HgTe. An intensity spectrumis recorded in the time domain by sequentially switching on and off ormodulating one or more of the sub-light sources 11′.

A more detailed depiction of the spectrometer of FIG. 1 can be seen inFIG. 3 , wherein a pulsed and staggered control of the sub-light sources11′ is used. Since the sub-light sources 11′ are operated in pulsedmode, the secondary light 13 emitted by the arrays 12 is also emitted ina pulsed manner, which then leads to the transmitted light 31 also beingemitted in a pulsed manner and thus being detected by the detector 32 ina pulsed manner. This makes it possible to activate the arrays 12sequentially. The output of the detector 32 is converted into a digitalsignal 34 via an amplifier and an analogue-to-digital converter (ADC)33, which is in turn emitted, for example, to a microcontroller, afield-programmable gate array (FPGA) or a digital signal processor (DSP)35. This now outputs spectral information, specifically the spectrum 36,which is the end result of this analysis. The microcontroller/DSP/FPGA35 also serves to emit control signals 37 to the light source 1 and thesub-light sources 11′. The microcontroller/DSP/FPGA can furthermoreinterpret the spectrum and display any information regarding the sampleproperties to the user. The spectrometer can furthermore be coupled to acomputer or network and be used, for example, to monitor (automated)processes.

A further embodiment is shown in FIG. 4 . As far as control and analysisare concerned, this embodiment functions in substantially the samemanner as the spectrometer shown in FIGS. 1 and 3 , which is why adescription of the identical aspects is not provided here. Substantiallythe same reference numbers as in FIG. 1 are used here, with 100 beingadded in each case.

The spectrometer 100 shown in FIG. 4 is a reflectance spectrometer thatrecords a reflection spectrum of the sample 121. It should be noted thatneither the angle of incidence of the secondary light on the sample northe exit angle are fixed and are arbitrarily selected. The same alsoapplies to the other dimensions and angles in the embodiments. It shouldfurthermore be noted that (for instance in the case of scattering on arough surface) the angle of incidence and the angle of reflection do nothave to be identical. A light source 111 consists of a plurality of (inthe present case: four) sub-light sources 111′, on each of which anarray 112 of quantum dots is provided. Light emitted by the sub-lightsources 111′ is incident on these arrays 112, where it is converted intosecondary light 113 that differs in wavelengths λ₁ to λ₄ from thewavelengths emitted by the four different sub-light sources 111′. Thesecondary light 113 is incident on the sample 121 and is reflectedthere. The reflected light 131 then impinges on the detector 132. Thelight or light intensity is measured there, so that a reflectionspectrum can be created in the same way as described above in connectionwith FIG. 3 . The spectrometer according to the second embodiment of theinvention, as schematically shown in FIG. 4 , can thus generate areflection spectrum.

There are, in general, various advantages to using quantum dots. Forexample, as is also shown in FIG. 2 , a size-quantisation effect occurswhen using nanoscopic semiconductor particles, i.e. quantum dots. Theband gap of the semiconductor material depends on the particle size. Theelectronic transitions leading to photoluminescence are thus alsodependent on this particle size. Since nanoparticles can essentially nowbe produced in any size, it is therefore also possible to adjust theemission wavelength of the quantum dots almost infinitely. This is asignificant advantage over the use of conventional inorganic or organicphosphors and emitters.

Quantum dots furthermore have a large Stokes shift. This means thatparticles with different emission wavelengths of the secondary light canbe excited with primary light of the same wavelength. This simplifiesthe structure of the light source since a homogeneous LED array, forexample, can be used as the primary light source.

The quantum dots can also be applied in the form of inks usingcomparatively simple processes such as spin coating, dip or dropcoating, or printing processes based on inkjet printing on the primarylight emitter or on a suitable substrate that can be applied to aprimary light emitter.

The quantum dots with different emission wavelengths also behavesubstantially similarly with respect to the envisioned coatingprocesses. The selection of the nanoparticles and the emissionwavelengths and thus the analysis wavelengths of the spectrometer canthus be adapted for the given case of use without having to interveneheavily in the production process. The spectrometers can therefore beeasily adapted to the issue to be examined.

Due to the similar behaviour of different QDs, the QD arrays may alsoconsist of mixtures of QDs so that the emission behaviour thereofresembles the absorption behaviour of potential target analytes, andthese can be detected more efficiently. It is extremely difficult togenerate multi-band emission spectra with individual conventional LEDsor lasers. The use of QD arrays with multiple matched emission bandsthus leads to savings in terms of space/cost and resources.

Quantum dots furthermore have a high photoluminescence quantum yield ofup to 1, which results in high light intensity. Quantum dots are thussuperior to organic fluorophores, in particular in the near-infraredrange.

There are therefore numerous advantages which lead to a spectrometersuch as shown in FIGS. 1, 3 and 4 being able to be used in a wide rangeof applications. These include in particular the identification,analysis and quality assurance of food, products, materials, rawmaterials, arable soils and waste products, etc. Such a spectrometer canfurthermore also be used in medical diagnostics. Due to the possibleminiaturisation, portable and configurable spectrometers can bedeveloped which a user can use on site. It is also possible, inprinciple, to incorporate such a miniaturised spectrometer into aportable device such as a tablet or mobile phone. Even though thedescribed spectrometers lead to advantages in particular in thenear-infrared range, they can also be used in the visible andultraviolet range.

1. A spectrometer comprising: a plurality of arrays of quantum dots, thearrays being arranged such that primary light emitted by a light sourceis incident thereon and excites the quantum dots such that the quantumdots emit secondary light such that the secondary light can be incidenton a sample, the secondary light having a different spectraldistribution to the primary light, at least one detector configured toreceive the secondary light reflected or transmitted by the sample, andan evaluation device configured to determine spectral information,preferably a spectrum, of the secondary light received by the at leastone detector.
 2. The spectrometer according to claim 1, furthercomprising a light source arranged such that primary light emitted bysaid light source is incident on the arrays of quantum dots.
 3. Thespectrometer according to claim 1, wherein at least two of the pluralityof arrays of quantum dots have different emission spectra, the particlesizes and/or the materials of the quantum dots in at least two of theplurality of arrays of quantum dots preferably differing from oneanother.
 4. The spectrometer according to claim 3, wherein thespectrometer is configured such that the secondary light emitted by thearrays of quantum dots with different emission spectra can be modulated,with preferably only one of the arrays of quantum dots with differentemission spectra at a time emitting secondary light such that it can beincident on the sample.
 5. The spectrometer according to claim 4,further comprising a control device for controlling from which of thearrays of quantum dots secondary light is incident on the sample.
 6. Thespectrometer according to claim 5, further comprising a light sourcearranged such that primary light emitted by said light source isincident on the arrays of quantum dots, wherein the light sourcecomprises a plurality of sub-light sources, each of which isrespectively coupled to one of the arrays of quantum dots such thatlight emitted by the respective sub-light source is incident on therespective array, the control device being configured such that it cancontrol from which of the sub-light sources primary light is emitted, orwherein the light source comprises a plurality of selectivelylight-transmissive windows controlled by the control device such thatthey selectively allow light from a single primary light source to passthrough, the light that is allowed to pass through the windowsrespectively being substantially incident on only one singlecorresponding array, and/or wherein a plurality of selectivelylight-transmissive windows are provided that are controlled by thecontrol device such that they selectively allow secondary light from thearrays to pass through, the secondary light being generated by primarylight from a single primary light source, wherein the windows preferablycomprise LCDs and/or shutters and/or micro-opto-electro-mechanicalmembers.
 7. The spectrometer according to claim 1, wherein thespectrometer is configured such that primary light from a light sourceexternal to the spectrometer can be incident on the arrays, wherein thespectrometer furthermore comprises a device configured to control fromwhich of the arrays secondary light is incident on the sample, whereinthe device preferably comprises components that are configured toselectively prevent primary light from the light source from beingincident on one or more of the arrays and/or wherein the devicepreferably comprises components configured to selectively preventsecondary light emitted by the arrays from being incident on the sample,wherein the components even more preferably comprise shutters and/or LCDmembers and/or micro-opto-electro-mechanical members.
 8. Thespectrometer according to claim 1, wherein quantum dots having differingemission spectra are present in at least one of the plurality of arrays.9. The spectrometer according to claim 1, wherein at least some of thequantum dots are configured such that they emit light in thenear-infrared range.
 10. The spectrometer according to claim 1, whereinthe wavelength of the primary light is shorter than the wavelength ofthe secondary light.
 11. The spectrometer according to claim 1, whereinthe quantum dots are embedded in a matrix, preferably a polymer matrixor a matrix made of an inorganic material.
 12. The spectrometeraccording to claim 2, wherein the quantum dots are arranged on a surfaceof the light source.
 13. The spectrometer according to claim 1, whereinthe quantum dots of one or more of the arrays consist of a plurality ofmaterial domains, the material domains preferably having a core-shellconfiguration or a Janus configuration.
 14. The spectrometer accordingto claim 1, wherein the spectrometer is configured such that primarylight is incident on the plurality of arrays of quantum dots in amodulated, preferably pulsed, mode.
 15. The spectrometer according toclaim 1, wherein the quantum dots provided in at least one of the arraysof quantum dots are identical to one another.
 16. The spectrometeraccording to claim 1, wherein the quantum dots in at least one of thearrays consist of quantum dots made of a semiconductor, preferably aIV-VI, II-VI, III-V or I-III-VI semiconductor, more preferred a IV-VIsemiconductor, even more preferred a lead chalcogenide or lead sulphide.17. A method for producing a spectrometer according to claim 1, whereinthe arrays of quantum dots are produced by way of spin coating, dipcoating, drop coating or a printing process.